The present invention relates to a holographic ROM (read-only memory) system; and, more particularly, to the holographic ROM system capable of achieving an enhanced recording and reading efficiency with a simplified structure.
Conventional holographic memory systems normally employ a page-oriented storage approach. An input device such as a SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as a CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-oriented approach. All of these systems, however, suffer from a common drawback in that they require the recording of a huge number of separate holograms in order to fill the memory to capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another conventional holographic ROM system such as FIG. 1 has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.
The conventional holographic ROM system of FIG. 1 includes a light source 100, HWPs (half wave plates) 102, 112, an expanding unit 104, a PBS (polarizer beam splitter) 106, polarizers 108, 114, mirrors 110, 116, a mask 122, a hologram medium 120, and a conical mirror 118.
The light source 100 emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is of only one type of linear polarization, e.g., P-polarization or S-polarization, is provided to the HWP 102. The HWP 102 rotates the polarization of the laser beam by xcex8 degree (preferably 45xc2x0). And then, the polarization-rotated laser beam is fed to the expanding unit 104 for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS 106.
The PBS 106, which is manufactured by repeatedly depositing at least two kinds of materials each having a different refractive index, serves to transmit one type of polarized laser beam, e.g., P-polarized beam, and reflect the other type of polarized laser beam, e.g., S-polarized beam. Thus the PBS 106 divides the expanded laser beam into a transmitted laser beam (hereinafter, a signal beam) and a reflected laser beam (hereinafter, a reference beam) having. different polarizations, respectively.
The signal beam, e.g., of a P-polarization, is fed to the polarizer 108, which removes imperfectly polarized components of the signal beam and allows only the purely P-polarized component thereof to be transmitted therethrough. And then the signal beam with perfect or purified polarization is reflected by the mirror 110. Thereafter, the reflected signal beam is projected onto the holographic medium 120 via the mask 122. The mask 122, presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM).
On the other hand, the reference beam is fed to the HWP 112. The HWP 112 converts the polarization of the reference beam such that the polarization of the reference beam becomes identical to that of the signal beam. And then the reference beam with converted polarization is provided to the polarizer 114, wherein the polarization of the reference beam is more purified. And the reference beam with perfect polarization is reflected by the mirror 116. Thereafter, the reflected reference beam is projected onto the conical mirror 118 (the conical mirror 118 being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The reflected reference beam is reflected toward the holographic medium 120 by the conical mirror 118. The incident angle of the reflected reference beam on the holographic medium 120 is determined by the base angle of the conical mirror 118.
The holder for fixing the conical mirror 118 should be installed on the bottom side of the conical mirror 118, in order to prevent the reference beam from being blocked by the holder. Since the holder should be placed on the bottom side of the conical mirror 118, it is usually installed through a center opening 124 of the holographic medium 120.
The holographic medium 120 is a disk-shaped material for recording the data patterns. The disc-sized mask 122 provides the data patterns to be stored therein. By illuminating the mask 122 with a normally incident plane wave, i.e., the signal beam, and by using the reference beam incident from the opposite side to record holograms in the reflection geometry, the diffracted pattern is recorded in the holographic medium 120. A conical beam shape is chosen to approximate the plane wave reference beam with a constant radial angle at all positions on the disc, such that the hologram can be read locally by a fixed-angle narrow plane wave while the disc is rotating during playback. Furthermore, an angular multiplexing can be realized by using the conical mirror 118 with a different base angle (see xe2x80x9cHolographic ROM system for high-speed replicationxe2x80x9d, 2002 IEEE, by Ernest Chuang, et al.).
By using the above-mentioned scheme, the time required to produce a fully recorded 100 GB-order capacity disc may be reduced to less than a minute, and potentially to an order of seconds.
However, there are critical problems in the prior art system;
First, the conventional holographic ROM system of FIG. 1 has a complicated structure. That is, the conventional holographic ROM system requires numerous components including the PBS 106, the HWPs 102, 112, the mirrors 110, 116, and the polarizers 108, 114 increasing the manufacturing cost thereof.
Secondly, replacement of the conical mirror 118 for angular multiplexing can be highly cumbersome. Since the holder is packed into a compact space between the holographic medium 120, the conical mirror 118 and the mask 122, the alignment between the holographic medium 120 and the mask 122 can be easily disturbed whenever the conical mirror 118 is replaced for the angular multiplexing, complicating the replacement process of the conical mirror 118.
It is, therefore, an object of the present invention to provide a holographic ROM system with a simplified structure, which also enables a conical mirror to be replaced easily for angular multiplexing.
In accordance with the present invention, there is provided a holographic ROM system including: a light source for emitting a laser beam; an expanding unit for expanding the laser beam; and a mask for modulating a certain portion of the expanded laser beam to thereby generate a signal beam which is directly provided to a holographic medium and sifting the remainder portion of the expanded laser beam to thereby generate a reference beam which is provided to a conical mirror reflecting the reference beam toward the holographic medium, wherein the holographic medium includes: a recording region onto which both the reference beam and the signal beam are projected; and a transparent region which passes through the reference beam sifted by the mask, to thereby provide the reference beam to the conical mirror.